Electrical control for vacuum pump

ABSTRACT

An electrical control for operating a vacuum powered pumping device which supplies fluid under pressure to operate a spring braking device. A pressure operating signal is communicated to the electrical control and an indicator to inform the operator of the operational pressure mode in the supply system. If a low pressure mode signal is generated, a temperature responsive switch sequentially interrupts the electrical energy flow to the electrical control. By interrupting the electrical energy flow to the electrical control, the pumping device will reciprocate and increase the pressure in the system causing the pressure mode signal to change. With a change in the pressure mode signal indicating a higher pressure, the electrical energy flow to the control will be terminated.

United States Patent Cripe Aug. 29, 1972 [54] ELECTRICAL CONTROL FORVACUUM PUMP [72] Inventor: Maxwell L. Cripe, South Bend, Ind. [73]Assignee: The Bendix Corporation [22] Filed: March 19, 1971 [21] Appl.No.: 125,978

[52] US. Cl. ..60/51, 60/54.5 P, 60/60, 60/545 E [51] Int. Cl. ..Fl5b1/02 [58] Field of Search ..60/51, 54.5 P, 54.5 E, 60

[56] References Cited UNITED STATES PATENTS 2,293,542 8/1942 Hamilton..60/60 X 2,323,519 7/1943 Dean ..60/60 UX 2,705,870 4/1955 Holton..60/60 UX Primary Examiner-Edgar W. Geoghegan Attorney--Leo H.McCormick, Jr. and Plante, l-lartz, Smith and Thompson [57] ABSTRACT Anelectrical control for operating a vacuum powered pumping device whichsupplies fluid under pressure to operate a spring braking device. Apressure operating signal is communicated to the electrical control andan indicator to inform the operator of the operational pressure mode inthe supply system. lf a low pressure mode signal is generated, atemperature responsive switch sequentially interrupts the electricalenergy flow to the electrical control. By interrupting the electricalenergy flow to the electrical control, the pumping device willreciprocate and increase the pressure in the system causing the pressuremode signal to change. With a change in the pressure mode signalindicating a higher pressure, the electrical energy flow to the controlwill be terminated.

10 Claims, 1 Drawing figure ELECTRICAL CONTROL FOR VACUUM PUMPBACKGROUND OF THE INVENTION Spring brakes have been proposed as one partof the dual braking systems required by the Federal Highways SafetyLaws. Initially, the spring brake was operated by an inversion valveconnected to the same source of pressurized fluid used to operate theentire braking system. If, for some reason the source of pressurizedfluid was lost, the spring brakes were automatically applied, asdisclosed in copending U.S. application Ser. No. 797,530, filed Feb. 7,1969, now US. Pat.

No. 3,599,761 owned by the same assignee and incorporated by reference.Later as disclosed in copending.

US. application Ser. No. 28,843, filed Apr. 15, 1970, now U.S. Pat. No.3,617,097, owned by the same assignee and incorporatedby reference, asplit fullpower system having an inversion valve capable of beingmodulated was developed to control the actuation of the spring brakes.In these split systems a pump driven by the crankshaft supplied thefluid under pressure to operate the spring brakes. Unfortunately, inmodern automobiles the equipment driven by the crankshaft is everincreasing, i.e., air conditioning, power steering, fuel pump etc., witha resulting smaller crankshaft power output. In addition, theavailablepower output has been further reduced partially by the use ofunleaded gas to reduce pollution and the available engine space underthe hood.

With further development it was discovered that a vacuum driven pumpcould be operated to supply the necessary fluid under pressure tooperate the spring brakes. A pressure indicator in the supply line tothe spring brakes would inform the operatorof the pressure modeavailable to operate the spring brakes. In US. application Ser. No.126,020, filed Mar. 19, 1971, owned by the common assignee of thisinvention and incorporated by reference if a low pressure mode wasindicated by the signal generated, the operator manually modulated acontrol valve to operate the vacuum driven pump to build up the pressureof the fluid in the supply line. But since, this operation requires anovert act on the part of the operator, the pressure differentialcontrolled vacuum pump, as disclosed in application Ser. No. 91,641,filed Nov. 23, 1970 owned by the common assignee of this invention andincorporated herein by reference, was developed. The force created bythe pressure differential across a movable wall is continually comparedwith the force required to move a piston in the pump which pressurizesthe fluid going to the supply line. However, as shown by the pressureindicator, the vacuum pump need only be operational during periods whena low pressure mode exists in the supply system.

SUMMARY OF THE INVENTION It has been observedthat as long as internalcombustion engines are running, vacuum will be produced at the intakemanifold. The intensity of the vacuum will normally be thegreatest whenthe accelerator pedal is released, as during periods of braking. This isbecause pump for supplying high pressure fluid to operate the auxiliaryspring brakes. The vacuum driven pumping device has a housing whoseinterior is divided by a diaphragm. A piston is secured to a push rod topressurize fluid in the supply line in response to movement by,thediaphragm. One side of the diaphragm of the pumping device is maintainedunder atmospheric pressure while vacuum and atmospheric pressure arealternately communicated to the other side of the diaphragm by anelectrically. operated control valve. Through this alternatecommunication a pressure dif ferential will be created across thediaphragm which will cause the-piston to reciprocate. The piston willautomatically reciprocate to pressurize fluid communicated by a conduitto an accumulator until a signal representing a predetermined pressuremode interrupts the flow of electricity to a temperature responsiveswitch which alternatively energizes the electrical control valve. Acheck valve in the conduit willprevent backflow of the fluid transmittedto the accumulator. At the bottom of the power stroke atmosphericpressure will be supplied to both sides of the diaphragm to permit aspring acting on the diaphragm to move the piston to the top of thepower stroke. In this position upon energizing the electrical controlvalve, vacuum will evacuate the front chamber to create the operationalpressure differential. A manually operated inversion valve has an inletattached to the accumulator which normally permits fluid pressure topass to the spring brakes through a control port. When an operatordesires to activate the spring brakes, the inversion valve is moved toinhibit the flow of fluid from the inlet while permitting fluid flowfrom the control port through an. outlet to a reservoir operativelyconnected to the inlet port of the pumping piston. During periods that'alow pressure mode exists in the supply conduit, a signal will betransmitted to an indicator to alert the operator of this condition.

It is therefore an object of this invention to provide a power brakingsystem with an electrically controlled vacuum pumping means foroperating a spring brake means.

stant source of high pressure to operate a spring brake means.

These and other objects of my invention will become apparent fromreading the specification and viewing the drawing.

BRIEF DESCRIPTION OF THE DRAWING In the drawing an enlarged sectionalview of an electrical control valve for operating a vacuum pump whichsupplied fluid pressure to operate a part of a dual braking system isshown.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT In the drawing there isrepresented a dual full power braking system with the main system Abeing responsive to the split master cylinder and the auxiliary system Bbeing responsive to the inversion valve 12. Each system is operatedindependently of the other even though a common reservoir 14 in themaster cylinder 10 is used to supply the same type of fluid to bothsystems.

In the main braking system A, in response to the application of a brakepedal (not shown) a piston (not shown) in the split master cylinder 10will simultaneously transmit a pressure signal to the front power brakeservomotor 16, of a type which operates in a manner fully described inU. S. Pat. No. 3,108,615 owned by the common assignee of the applicationand incorporated herein by reference, to operate the front servicebrakes 18 (only one of which is shown) and to the rear tandem powerbrake servomotor 20, which operates in the same manner as the frontpower brake servomotor 16, for operating the rear service brake 22,(only one of which is shown) in a manner fully described in copending U.S.' application Ser. No. 797,530, filed Feb. 7, 1969, now U. S. Pat. No.3,599,761 incorporated herein by reference.

In the braking auxiliary system B used for emergency and parking, aninversion valve 12, of a type disclosed in copending U. S. applicationSer. No. 28,843 filed Apr. 15, 1970, now U. S. Pat. No. 3,617,097,incorporated by reference above, controls fluid pressure being suppliedto a spring brake 24 operating in a manner fully described in U. S.application Ser. No. 797,530 filed Feb. 7, 1969, now U. 8. Pat. No.3,599,761, and incorporated herein by reference. A vacuum pumping device26, operatively connected by conduit 28 to accumulator 30 willpressurized fluid received from reservoir 14. Through the accumulator 30the fluid pressure acting on the spring brake 24 will be maintainedwithin relatively constant range. A switch 32, of a type fully describedin U. S. application Ser. No. 784,775 filed Dec. 18, 1968, now U. S.Pat. No. 3,593,265, owned by the same common assignee of thisapplication and incorporated herein by reference, is located in conduit28 adjacent the accumulator 30. Switch 32 has an internal movable shaft(not shown) which is responsive to the fluid pressure in the conduit. Ifa low pressure is present in the conduit 28, the shaft will move toclose an electrical contact switch which will cause a signal to be sentto an indicator device 34, either a warning light (as shown), whichcontinually glows or flashes or an audible signal (not shown), to beenergized for alerting an operator of this pressure mode condition.Electrical energy representative of this signal indicating an existingpressure mode will flow to a temperature responsive switching means 36which will sequentially interrupt the continued flow to an electricallyoperated control valve means 38. The control valve means 38 willperiodically be energized by the electrical energy flow causing thevacuum pumping device 26 to pressurize the operational fluid in thesupply conduit 28.

In more particular detail the vacuum operated pumping device 26 consistsof a power transmitting chamber 39 and a fluid intensifying chamber 42connected to the spring brake means 24.

The power transmitting chamber 39 has a housing 44- with an internalcavity. A wall of diaphragm means 46 has a first bead 58 held by anannular rib 60 in the housing and a second bead 54 held to a stiffinternal two piece plate 52 which is clamped together to divide thecavity into a front chamber 48 and a rear chamber 50. The front chamberhas an opening 49 through which the electrically operated control valvemeans alternatively ports vacuum and atmospheric pressure while the rearchamber 50 has an opening 47 covered by filter 51 through whichatmospheric pressure flows unrestricted into the rear chamber 50. Aflexible portion 56 located between the first and second beads 58 and 54will permit the wall or diaphragm means 46 to be freely moved axiallywithin the cavity. A resilient member 62 is concentrically located onplate 52 to surround a push rod 64 attached to plate 52. A pair of nuts66 and 68 are threaded on the push rod 64 in such a manner as to holdthe two piece plate 52 together. The push rod 64 extends through thehousing 44 into the intensifying chamber 42 to transmit any forcecreated by movement of the diaphragm member 46 caused by a pressuredifferential between the front chamber and the rear chamber to becommunicated thereinto.

The fluid intensifying chamber 42 has a cylindrical body 70 with anaxially extending chamber 72 from which fluid is forced through outlet74 by displacement piston means 40 attached to push rod 64. To replacethe loss of fluid in chamber 72 displaced by piston means 40 moving tothe right to transmit pressurized fluid to the accumulator 30, acompensating port 76 is connected to inlet port 78 in communication withreservoir 14. The change in volume of fluid in chamber 72 occurs becausecheck valve 80 prevents the back flow of fluid as the piston means 40returns to the left to the top of the power stroke. During poweractivation, the compensation port 76 is closed by a poppet member 82retained in an enlarged section 84 of the piston 86 seating on shoulder88. The push rod 64 to which poppet member 82 is attached, projects intothe enlarged section 84 and is loosely fastened to the piston 86 bycross pin 90. The cross pin 90 is fixed to the push rod 64 but moves ina slotted section 92 on the piston 86. The length of the slotted section92 is designed to permit the poppet member 82 to be unseated when thepin engages the rear end of the slot and seated on shoulder 88 at thefront end of the slot. The cross pin 92 is prevented from coming looseby a snap ring 94 positioned in a groove on the outer surface of thepiston 86 overlying the ends of cross pin 92. A guide bearing 96,retained by a snap ring 98, maintains push rod 64 in alignment with theintensifying chamber 42 and prevents any fluid from entering into thefront chamber 48.

The temperature responsive switching means 36 consists of a housinghaving a first terminal 102 connected to the pressure responsive switch32 and a second terminal 104 connected to the electrically operatedcontrol valve means 38. The first terminal 102 has a contact strip 106ending in a contact point 108. A first strip of metal 1 10 having oneend fixed to a non-conductive projection l 12 on the housing 100 by apair of pins 114 and 116 has a contact point 118 which is adjacentcontact pin 108. A second strip of metal 120 having a differentcoefficient of expansion than the first strip is fixed to the firststrip 1 10. The and second strips of metal bias the contact point 118into contact 108 to close the circuit between pin 116 and first terminal102. A heating wire 124 which is attached to the first strip of metaladjacent contact 118 coils around the first and second metal stripsuntil it is connected to pin 114. A contact strip 122 connected to pin116 and the second terminal 104 will complete the electrical circuitthrough the switching means 36.

The electrically operated control valve means 38 consists of a housing130 having an internal chamber 132 with an inlet port 134 connected toatmospheric pressure through filter 136, an outlet port 138 connected tothe manifold 140 of an internal combustion engine and a control port 142connected to the front chamber 48 of the pumping device 26.

A plunger 144 having one end with an annular face 146 which is locatedadjacent the outlet port 138 and another end 150 surrounded by a coil152 is held in chamber 132 by a spidered bearing surface 148. The coil152 receives electrical energy from the second terminal 104 throughcontact 156 and is grounded at 158. A resilient member 160 is cagedbetween the wall 162 and a keeper 164 on the plunger shaft to urge face146 into seating on ribs 166 to close the communication through theoutlet port 138.

MODE OF OPERATION OF THE PREFERRED EMBODIMENT After turning on ignitionswitch 168 shown in the drawing, if low pressure exists in theaccumulator 30 and it is insufficient to release the spring brakes, apressure mode signal will be indicated through light 34. This will alertthe operator not to try to attempt to move the vehicle until the fluidpressure in accumulator 30 is raised to release the spring brakes 24.

When the engine of the vehicle is started, vacuum will be created at themanifold 140. This vacuum will be carried through conduit 172 to be madeavailable at the outlet port 138 of the control valve means 38. Sinceresilient member 160 maintains face 146 in a seated position over outletport 138 atmospheric pressure freely flowing through inlet port 136 intochamber 132 through the spidered bearing 148 around seat 180 and out thecontrol port 142 and by way of conduit 174 into the front chamber 48where resilient member 62 urges the diaphragm means 46 toward the rearchamber 50.

At the same time the pressure mode signal is transmitted to theindicator 34 an electrical energy signal representative of the pressuremode signal will be carried through lead 170 to the first terminal 102of switching means 36. This electrical energy flows through contactpoints 108 and 118, bi-metal strip 110, 120 out the second terminal tothe coil 152 of the control valve means 38. The electrical energyreceived by the coil 152 will create a magnetic attraction which willmove the plunger 144 overcoming resilient means 160. The plunger 144will move to unseat face 146 from seating on ribs 166 thus connectingcontrol port 142 with outlet 138 and seat face 147 on the annularprojection 180 to close atmospheric pressure communication of the inletport 136 and the control port 142. In this position, vacuum communicatedthrough outlet port 138 will evacuate the front chamber 48 to create apressure differential across the wall means 46. This pressuredifferential acting on wall means 46 will generate a force which willmove push rod 64, closing compensating port 76 and pressurize the fluidin the chamber 72 passing through outlet port 74 into supply conduit 28.

Simultaneously with this operation in the pumping device 26, some of theelectrical energy flowing through switching means 36 is converted tothermal energy by heating coil 124 resisting the electrical flow. Theincrease in thermal energy causes the bi-metal strip 110, to expanddifferently and break the contact points 108, 118 and interrupt theelectrical energy flow therethrough. With the flow of electrical energyto coil 152 terminated, resilient member again moves plunger 144 to seatface 146 on ribs 166 closing the outlet port 138 and opening inlet 'port134 to allow atmospheric pressure to travel through the control port 142and into the front chamber 48 of the vacuum pumping device 26. Withatmospheric pressure in both chambers 48 and 50, the resilient member 62will now urge the diaphragm means 46 toward the rear chamber 50 andreturn the piston 86 to the up stroke position completing a singlecycle. As the piston 86 is being returned, compensating port 76 isopened replacing the pressurized fluid transmitted to the accumulator30. The above cycle is repeated by the temperature sensitive switchingmeans 36 operating the electrically operated control valve means 38until the pressure mode signal from switch 32 breaks the electricalenergy circuit rendering the control valve means 38 inoperative. In theinoperative state, atmospheric pressure is freely communicated to thefront 48 and rear 50 chambers of the pumping device 26 to permit theresilient member 62 to hold the piston means 86 at the top of the powerstroke in the rest position. At this point, the pressure in theaccumulator 30 will have reached a predetermined valve sufficient forthe switch 32 to transmit a different pressure mode signal to indicator34 causing the light to go out. This predetermined pressure available inthe supply line 28 will permit the spring brakes to be released inresponse to the operator controlled inversion valve 12.

As shown in the drawing, inversion valve 12 is in the position with thespring brakes 24 applied. In the applied position, fluid communicationfrom inlet port connected to the accumulator 30 is inhibited by a ballvalve 172 being urged against seat 174 by spring 176. Any fluid pressureacting on the spring brakes 24 can now escape through control port. 173through activation stern and out the outlet port 182 to the reservoir14.

When sufficient fluid pressure has been stored in the accumulator 30,upon moving the activation stem 180, by positioning the manual levermeans 184 to the dashed location as shown in the drawing, the ball valve172 will be unseated. With the activation stern 180 seated on ball valve172, pressurized fluid flow will pass between the stem 180 and the seat174 to be communicated through control port 173 to release the springbrake means 24.

In the event that a malfunction should occur in the system with fluidbeing lost or unavailable at inlet port 78 or vacuum not being producedat manifold 140, the vacuum pumping device 26 would continue to operatebut would not change the operational pressure mode in the accumulator30. By designing the size of the accumulator to permit from two to fivemanual applications of the spring brakes 24 would permit the operator tomove a vehicle from the roadway to a shoulder and out of the immediateline of traffic if this type of malfunction should occur. When the fluidin the accumulator is depleted, the spring brakes will be automaticallyapplied. This will prevent movement of the vehicle until the springbrakes are released by fluid pressure or through manual means of a typedisclosed in U. S. application Ser. No. 38,088 filed May 18, 1970, nowU. S. Pat. No. 3,647,030, owned by the same assignee of this applicationand incorporated herein by reference. Thus, I have devised a brakingsystem which will operate independently of the crankshaft power producedto effectively produce sufficient energy to maintain an emergency orparking brake system.

I claim:

1. In a power braking system having a fluid operated spring brake meansfor use as an emergency and parking brake with pumping means forsupplying fluid under pressure to control the operation of said springbrake means, said pumping means comprising:

a housing having an internal cavity;

wall means dividing said cavity into a front chamber and a rear chamber,said rear chamber being continually subjected to atmospheric pressure;

a cylinder having a stepped bore with an inlet port and an outlet port,said inlet port being connected to a reservoir containing said fluid,said outlet port being connected by a conduit to said spring brakemeans;

piston means located in said stepped bore of the cylinder and connectedto said wall means;

resilient means located in said front chamber for urging said wall meanstoward said rear chamber; and

electrical control means responsive to the pressure of the fluid in saidconduit for alternately permitting vacuum and atmospheric pressure tosaid front chamber to create a pressure differential across said wallmeans causing said piston means to reciprocate in said bore andpressurize the fluid admitted to said cylinder through said inlet byforcing the fluid through said outlet and into the conduit to providethe operating fluid force for said spring brake means.

2. The power braking system, as recited in claim 1 including:

means responsive to the fluid pressure being supplied to said springbrake means for transmitting a signal to an indicator device and saidelectrical control means representing the intensity of the pressure modeexisting in the supply conduit.

3. The power braking system, as recited in claim 2 wherein saidelectrical control means includes:

a housing having a chamber with a vacuum port, an atmospheric pressureport and a control port, said control port being connected to the frontchamber of the housing of the pumping means;

electrically operated valve means for alternatively opening and closingsaid control port to said vacuum port and said atmospheric pressure portcausing said pressure differential across the wall means; and

switching means connected to said electrically operated valve means forreceiving said pressure mode signal to sequentially energize saidelectrically operated valve means.

4. The power braking system, as recited in claim 3 wherein saidswitching means includes:

first means for receiving electrical energy represent ing said pressuremode signal;

second means for transmitting electrical energy to said electricallyoperated valve means in a predetermined time sequence;

a first strip of metal attached to said second means and in contact withsaid first means, said contact permitting electrical energy to flow fromsaid first means to said second means;

heating means operated by the flow of electrical energy through saidfirst strip of metal, said heating means raising the temperature of themetals, said first strip of metal expanding when subjected to saidtemperature change causing the contact between said first strip of metaland said first means to be broken and interrupting the flow ,ofelectrical energy therebetween, said interrupted energy flow to saidheating means permitting the temperature of the dissimilar metals to belowered to allow contact between said first strip of metal and saidfirst means.

5. The power braking system, as recited in claim 4 wherein saidelectrically operated valve means includes:

a shaft located in the chamber with a face on one end overlying saidvacuum port, the other end of the shaft extending through a bearing wallin the housresilient means secured to said shaft for urging said faceinto a seating position with said vacuum port to prevent communicationtherethrough; and

coil means surrounding said other end of the shaft connected to thesecond means of the switching means for overcoming the force of saidresilient means to allow communication through the vacuum port inresponse to the flow of electrical energy from said second means.

6. The power braking system as recited in claim 5 further including:

flow control means in the supply conduit adjacent the outlet port of thecylinder for preventing .backflow of fluid through the bore of thecylinder.

7. The power braking system as recited in claim 6, further including:

means connected to the supply conduit downstream from the flow controlmeans for storing a supply of fluid under pressure sufficient to permita series of spring brake releases without the pumping means beingenergized.

8. The power braking system as recited in claim 7 further including:

actuating means connected to the supply means responsive to an operatorfor permitting fluid under pressure to release said spring brake means.

9. The power braking system as recited in claim 2, wherein saidelectrical control means includes:

a housing having a control chamber connected to the front chamber of thepumping means, said control chamber being in communication with a sourceof vacuum and atmospheric pressures;

means responsive to temperature change for interrupting the electricalenergy flow representing the pressure mode signal; and

solenoid operated valve means secured to the housing and controlled bysaid interrupted electrical energy flow for sequentially subjecting saidcontrol chamber with vacuum and atmospheric pressure to create pulsatingfluid flow to said front chamber of the pumping means.

10. The power braking system as recited in claim 9,

wherein said temperature responsive means includes:

a first terminal connected in parallel with and receiving the sameelectrical signal as the indicator device informing the operator of thefluid pressure mode; v

a second terminal connected to an electrical coil of the solenoidoperated valve means;

a bimetal strip connected tosaid second terminal and biased into contactwith said first terminal; and

a heating coil connected to said second terminal and

1. In a power braking system having a fluid operated spring brake meansfor use as an emergency and parking brake with pumping means forsupplying fluid under pressure to control the operation of said springbrake means, said pumping means comprising: a housing having an internalcavity; wall means dividing said cavity into a front chamber and a rearchamber, said rear chamber being continually subjected to atmosphericpressure; a cylinder having a stepped bore with an inlet port and anoutlet port, said inlet port being connected to a reservoir containingsaid fluid, said outlet port being connected by a conduit to said springbrake means; piston means located in said stepped bore of the cylinderand connected to said wall means; resilient means located in said frontchamber for urging said wall means toward said rear chamber; andelectrical control means responsive to the pressure of the fluid in saidconduit for alternately permitting vacuum and atmospheric pressure tosaid front chamber to create a pressure differential across said wallmeans causing said piston means to reciprocate in said bore andpressurize the fluid admitted to said cylinder through said inlet byforcing the fluid through said outlet and into the conduit to providethe operating fluid force for said spring brake means.
 2. The powerbraking system, as recited in claim 1 including: means responsive to thefluid pressure being supplied to said spring brake means fortransmitting a signal to an indicator device and said electrical controlmeans representing the intensity of the pressure mode existing in thesupply conduit.
 3. The power braking system, as recited in claim 2wherein said electrical control means includes: a housing having achamber with a vacuum port, an atmospheric pressure port and a controlport, said control port being connected to the front chaMber of thehousing of the pumping means; electrically operated valve means foralternatively opening and closing said control port to said vacuum portand said atmospheric pressure port causing said pressure differentialacross the wall means; and switching means connected to saidelectrically operated valve means for receiving said pressure modesignal to sequentially energize said electrically operated valve means.4. The power braking system, as recited in claim 3 wherein saidswitching means includes: first means for receiving electrical energyrepresenting said pressure mode signal; second means for transmittingelectrical energy to said electrically operated valve means in apredetermined time sequence; a first strip of metal attached to saidsecond means and in contact with said first means, said contactpermitting electrical energy to flow from said first means to saidsecond means; heating means operated by the flow of electrical energythrough said first strip of metal, said heating means raising thetemperature of the metals, said first strip of metal expanding whensubjected to said temperature change causing the contact between saidfirst strip of metal and said first means to be broken and interruptingthe flow of electrical energy therebetween, said interrupted energy flowto said heating means permitting the temperature of the dissimilarmetals to be lowered to allow contact between said first strip of metaland said first means.
 5. The power braking system, as recited in claim 4wherein said electrically operated valve means includes: a shaft locatedin the chamber with a face on one end overlying said vacuum port, theother end of the shaft extending through a bearing wall in the housing;resilient means secured to said shaft for urging said face into aseating position with said vacuum port to prevent communicationtherethrough; and coil means surrounding said other end of the shaftconnected to the second means of the switching means for overcoming theforce of said resilient means to allow communication through the vacuumport in response to the flow of electrical energy from said secondmeans.
 6. The power braking system as recited in claim 5 furtherincluding: flow control means in the supply conduit adjacent the outletport of the cylinder for preventing backflow of fluid through the boreof the cylinder.
 7. The power braking system as recited in claim 6,further including: means connected to the supply conduit downstream fromthe flow control means for storing a supply of fluid under pressuresufficient to permit a series of spring brake releases without thepumping means being energized.
 8. The power braking system as recited inclaim 7 further including: actuating means connected to the supply meansresponsive to an operator for permitting fluid under pressure to releasesaid spring brake means.
 9. The power braking system as recited in claim2, wherein said electrical control means includes: a housing having acontrol chamber connected to the front chamber of the pumping means,said control chamber being in communication with a source of vacuum andatmospheric pressures; means responsive to temperature change forinterrupting the electrical energy flow representing the pressure modesignal; and solenoid operated valve means secured to the housing andcontrolled by said interrupted electrical energy flow for sequentiallysubjecting said control chamber with vacuum and atmospheric pressure tocreate pulsating fluid flow to said front chamber of the pumping means.10. The power braking system as recited in claim 9, wherein saidtemperature responsive means includes: a first terminal connected inparallel with and receiving the same electrical signal as the indicatordevice informing the operator of the fluid pressure mode; a secondterminal connected to an electrical coil of the solenoid operated valvemeans; a bimetal strip connected to said secoNd terminal and biased intocontact with said first terminal; and a heating coil connected to saidsecond terminal and adjacent said bimetal strip, said heating coil beingoperated by the electrical energy flow between said first and secondterminals, said heating coil connecting electrical energy to thermalenergy causing the bimetal strip to expand non-uniformly overcoming thebiasing contact with said first terminal and stopping the electricalenergy flow between the first and second terminals to interrupt theconversion of electrical to thermal energy, said bimetal strip uponcooling returning to biased contact with said first terminal permittingcontinued electrical flow.